Refs #2675:

The known file names that are too long are mapped into the renamed_files.json.
When the migration gets a file that is too long, it sets its destination with the target from renamed_files. If the file cannot be found in renamed_files an Warning is thrown -> this should be fixed on the spot.
During the file migration, the source is set as a alias, so the old links could still be functional.
Because the source may be too huge, the limit of the url_alias.alias field is set to 768 chars for now.

This pull request contain tests for:
* the alias schema change (size is set to 768)

This pull request does not contain tests for:
* the utility function
* the migration